U.S. Secretary of State John Kerry (L) meets Russia's foreign minister Sergey Lavrov during a bilateral meeting at the sidelines of the ASEAN foreign ministers meeting in Vientiane, Laos July 26, 2016.

U.S. Secretary of State John Kerry is still pursuing an agreement with Russia on military cooperation in the fight against Islamic State in Syria despite major setbacks and skepticism from other administration officials and U.S. allies, U.S. officials with knowledge of the talks said on Friday.

"We believe this approach is still worth pursuing," State Department spokesman John Kirby said in an email response to Reuters questions, adding: "But it remains to be seen whether or not we can get there."

Kerry has been pursuing a proposal that envisions resuscitating a Cessation of Hostilities agreement, creating a center where the two countries would share intelligence for targeting air strikes, and prohibiting the Syrian air force from attacking U.S.-backed rebel groups.

Instead, Syrian and Russian warplanes have continued to pound rebels who are assaulting government-held parts of the northern city of Aleppo in an effort to reopen supply lines into opposition-held areas.

The task of identifying acceptable rebel targets has grown harder since a major Islamic extremist group said it had cut its ties to al Qaeda. That is leading some rebels to join the renamed group and made it harder to target hardliners without hitting other units.

"We've been very concerned about the situation in Aleppo and we have made those concerns plain to Russian officials," said Kirby, who noted that Kerry had spoken to his Russian counterpart, Sergei Lavrov, in a phone call on Friday.

Speaking in Laos last week, Kerry said he hoped for an agreement early in August, but two U.S. officials said on Friday there had been "limited progress" toward a deal.

"Discussions will likely continue, but there is no illusion on how much can be achieved," said another U.S. official, speaking on condition of anonymity.

Shared distrust

While Kerry shares other officials' distrust of the Russians, according to several U.S. officials who spoke on condition of anonymity, he thinks the diplomatic channel must be kept open in case Russia can be persuaded to help end the violence in Syria, now in its fifth year.

U.S. President Barack Obama has supported Kerry's effort, but he, too, expressed concern on Thursday about Russia's commitment to ending the violence, saying he was under no illusions about Russia's motives and they would be put to the test.

"I'm not confident that we can trust the Russians and Vladimir Putin," he told reporters after a meeting with his national security team at the Pentagon. "We have to test whether or not we can get an actual cessation of hostilities that includes an end to the kinds of aerial bombing and civilian death and destruction that we've seen carried out by the Assad regime."

A senior U.S. official, who also spoke on condition of anonymity, said that during the meeting at the Pentagon questions were raised over whether to take Russia's word.

"There was an acknowledgement that we were not, nor should we, take the Russians at their word," the official told Reuters. "And if this moves forward we'll have to make sure it’s in the best interests of the cessation of hostilities."

"No doors are closed but nothing has been decided," the official added.

The State Department's Kirby said the test for Russia was whether it was willing to use its influence over Assad to stop the violence and support a political transition in Syria.

"The test is to see if Russia is really willing to use its influence on the Assad regime to observe the cessation of hostilities, to stop killing its own citizens, to improve the delivery of humanitarian aid, and eventually contribute to the political process," Kirby said.

But a second U.S. official said progress in the talks was for now being overtaken by the battle for Aleppo.

The rebels are trying to break through a strip of government-controlled territory in an effort to reconnect their area of control in the west of Syria with the encircled rebel sector of eastern Aleppo.

The second official said another major factor was that Jabhat Fateh al Sham, which until last week called itself the Nusra Front, al Qaeda’s Syrian affiliate, is leading the drive to break the government siege of opposition-held northern Aleppo, and its fighters have intermingled with other rebel groups.